modify map_reader en test=develop (#20229)

* modify map_reader en test=develop

paddle/fluid/API.spec

@@ -94,7 +94,7 @@ paddle.fluid.io.DataLoader.__init__
 paddle.fluid.io.DataLoader.from_dataset (ArgSpec(args=['dataset', 'places', 'drop_last'], varargs=None, keywords=None, defaults=(True,)), ('document', 'eb8b6d31e1c2ec2ca8ebbb62fcf46557'))
 paddle.fluid.io.DataLoader.from_generator (ArgSpec(args=['feed_list', 'capacity', 'use_double_buffer', 'iterable', 'return_list'], varargs=None, keywords=None, defaults=(None, None, True, True, False)), ('document', 'e3bdde36774236c3e381d2218e9cc09e'))
 paddle.fluid.io.cache (ArgSpec(args=['reader'], varargs=None, keywords=None, defaults=None), ('document', '1676886070eb607cb608f7ba47be0d3c'))
-paddle.fluid.io.map_readers (ArgSpec(args=['func'], varargs='readers', keywords=None, defaults=None), ('document', '77cbadb09df588e21e5cc0819b69c87d'))
+paddle.fluid.io.map_readers (ArgSpec(args=['func'], varargs='readers', keywords=None, defaults=None), ('document', '2d0903e1d2f00b4f1d6618e6b5310121'))
 paddle.fluid.io.buffered (ArgSpec(args=['reader', 'size'], varargs=None, keywords=None, defaults=None), ('document', '0d6186f109feceb99f60ec50a0a624cb'))
 paddle.fluid.io.compose (ArgSpec(args=[], varargs='readers', keywords='kwargs', defaults=None), ('document', '884291104e1c3f37f33aae44b7deeb0d'))
 paddle.fluid.io.chain (ArgSpec(args=[], varargs='readers', keywords=None, defaults=None), ('document', 'e0311508658a7e741fc39feea8be0ad2'))

python/paddle/reader/decorator.py

@@ -61,13 +61,33 @@ def cache(reader):
 def map_readers(func, *readers):
     """
     Creates a data reader that outputs return value of function using
-    output of each data readers as arguments.
+    output of each data reader as arguments.
 
-    :param func: function to use. The type of func should be (Sample) => Sample
-    :type: callable
-    :param readers: readers whose outputs will be used as arguments of func.
-    :return: the created data reader.
-    :rtype: callable
+    If input readers output the following data entries: 2 3,
+    and the input func is mul(x, y),
+    the output of the resulted reader will be 6.
+
+
+    Args:
+        func: a function to read data and compute result, the output of this function 
+              will be set as the output of the resulted data reader.
+        readers (Reader|list of Reader): list of readers whose outputs will be used as arguments of func.
+ 
+    Returns:
+        the resulted data reader (Reader)
+
+    Examples:
+
+        .. code-block:: python
+
+         import paddle.reader
+         d = {"h": 0, "i": 1}
+         def func(x):
+             return d[x]
+         def reader():
+             yield "h"
+             yield "i"
+         map_reader_result = paddle.reader.map_readers(func, reader)
     """
 
     def reader():